Digital subscriber line (DSL) technologies can provide relatively large bandwidth for digital communications over existing subscriber lines. When transmitting data over the subscriber lines, crosstalk interference can occur between the transmitted signals over adjacent twisted-pair phone lines, for example in a same or nearby bundle of lines. Crosstalk limits the performance of some DSL technologies, such as asymmetric DSL 2 (ADSL2) and very high bit rate DSL 2 (VDSL2). For example, significant levels of crosstalk can occur because of the relatively high frequencies used in VDSL2. The crosstalk is highly non-stationary and varies dramatically as lines within the binder are activated and deactivated. Such rapidly varying noise environment causes frequent re-initializations of the lines, loss of service, and low customer satisfaction.
One method for increasing the stability of digital subscriber lines is using virtual noise, a technique that limits the maximum bitloading allowed on each tone. Virtual noise is used to improve DSL stability by ensuring operation of a line at a bitloading level that can be sustained when worse noise conditions are encountered. The performance of a DSL is substantially dependent on the virtual noise configuration for the line. For example, if the virtual noise is set too low, the line becomes unstable. Alternatively, if the virtual noise is set too high, the line's data-rate is unnecessarily reduced. Finding an appropriate configuration for the virtual noise can be difficult due to a variety of practical issues, such as difficulties in line noise data collection, limitations on the number of line profiles in DSL access multiplexers (DSLAMs), finding appropriate balance between line stability and data-rate, and adaptability to changes in the noise environment.